1. Field of the Invention
The present invention relates to wireless telecommunications and, more particularly, to a method of selecting a carrier frequency for a mobile station to use when transmitting a call origination message.
2. Description of Related Art
Wireless telecommunications networks typically include a plurality of distinct wireless coverage areas, e.g., “cells” or “sectors,” within which mobile stations can wirelessly communicate with the network. In some cases, a base transceiver station (BTS) may provide a single wireless coverage area, e.g., a cell. More typically, however, a BTS provides multiple wireless coverage areas, e.g., by using directional antennas, to provide multiple sectors within a given cell.
Each sector may have allocated to it a distinct set of downlink channels that the BTS uses for transmitting signals to mobile stations and a distinct set of uplink channels that mobile stations may use for transmitting signals to the BTS. The downlink channels may include, for example, a pilot channel, paging channels, and traffic channels. The uplink channels may include, for example, access channels and traffic channels. The uplink and downlink channels may use different carrier frequencies. For example, in the case of code division multiple access (CDMA) wireless communication, the uplink channels for a given sector may all use the same uplink carrier frequency and may be distinguished by different codes. The downlink channels for the given sector may all use the same downlink carrier frequency, i.e., a carrier frequency that is different than that used for the uplink channels, and may be distinguished by different codes.
Increasingly, however, wireless telecommunications networks are using multiple frequency plans in some or all of their sectors in order to provide additional wireless capacity in those sectors. In particular, a given sector may have allocated to it a plurality of uplink carrier frequencies, each with associated uplink channels, and a plurality of corresponding downlink carrier frequencies, each with associated downlink channels. As a result, it is often useful to consider the wireless coverage provided by a network as being divided into a plurality of distinct “sector-carriers.” Each “sector-carrier” is associated with a particular geographic area, e.g., a cell or sector, and with a particular set of one or more carrier frequencies. For example, a sector-carrier may have a set of uplink channels that use a particular uplink carrier frequency and a set of downlink channels that use a particular downlink carrier frequency.
The number of different carrier frequencies that are available in a given sector may depend on the configuration of the BTS that provides the wireless coverage in that sector. For example, a BTS may be configured to communicate using two different uplink carrier frequencies and two different downlink carrier frequencies. In that case, each sector that is provided by that BTS may be associated with two sector-carriers: (i) one that uses a first uplink carrier frequency and a first downlink carrier frequency; and (ii) one that uses a second uplink carrier frequency and a second downlink carrier frequency. Moreover, these same carrier frequencies may be used by different BTSs in the network. Thus, two different sector-carriers may be distinguished on the basis of their geographic locations (e.g., they may be associated with different sectors) and/or on the basis of their carrier frequencies (e.g., they may be associated with the same sector but use different carrier frequencies).
A mobile station may be associated with a particular sector-carrier by using or monitoring one or more of its channels. For example, a mobile station that is engaged in a call may be associated with a particular sector-carrier by using an uplink and/or downlink traffic channel of that sector-carrier. When a mobile station is not engaged in a call, e.g., in an idle state, the mobile station may be associated with a particular sector-carrier by monitoring the paging channel of that sector-carrier for messages from the network. The sector-carrier with which a mobile station is associated may change. For example, a handoff from one sector-carrier to another may occur when the mobile station changes its location. Such handoffs may occur when the mobile station is in an idle state, or when the mobile station is engaged in a call.
Once associated with a particular sector-carrier, a mobile station typically uses the access channels of that sector-carrier to transmit messages to the network. For example, to originate a call, the mobile station would typically transmit a call origination message in an access channel of the mobile station's current sector-carrier, i.e., the sector-carrier with which the mobile station is associated in the idle state. Then, if the network receives and is able to validate the call origination message, the network may transmit a channel assignment message to the mobile station via a paging channel of that sector-carrier. However, if multiple carrier frequencies are available at that location, the network may also instruct the mobile station to begin using a different sector-carrier, such as a sector-carrier that uses different uplink and downlink carrier frequencies than the sector-carrier that the mobile station used to transmit the call origination message. For example, in response to a mobile station's call origination message, the network may transmit an extended channel assignment message (ECAM), instructing the mobile station to use a different sector-carrier, with different carrier frequencies, for the call.
As noted above, networks often provide multiple carrier frequencies in certain areas in order to increase the number of traffic channels available, and, thus, increase the network's capacity in that area. However, in order to realize the benefits of using multiple carrier frequencies when a mobile station tries to originate a call, the network must receive the call origination message in the original sector-carrier's access channel and the mobile station must receive the channel assignment message that the network transmits in the original sector-carrier's paging channel. If the original sector-carrier's RF conditions are inadequate, however, then the messages needed to originate the call may not be received. For example, the mobile station may be located in an area with low signal strength from the BTS, or the mobile station may be located in an area with high interference. If, as a result of such poor RF conditions, the network fails to receive the mobile station's call origination message or the mobile station fails to receive the network's channel assignment message, the mobile station's attempt to originate the call may be unsuccessful.
Accordingly, there is a need to provide ways of using multiple frequency plans more efficiently.